1. Field of the Invention
The present invention relates to an organic light emitting display device and a method of driving the same, and more particularly to an organic light emitting display device capable of compensating for threshold voltage of a driving transistor and a method of driving the same.
2. Discussion of Related Art
Recently, various flat panel display devices capable of reduced in weight and volume over cathode ray tubes have been developed. As the flat panel display device, there are a liquid crystal display device, a field emission display device, a plasma display panel, an organic light emitting display device, etc.
Among others, the organic light emitting display device displays images by using an organic light emitting diode generating light by means of recombination of electrons and holes. Such an organic light emitting diode has advantages of being driven with low power consumption and having rapid response speed.
FIG. 1 is a circuit diagram showing a pixel of a general organic light emitting display device. In FIG. 1, transistors included in pixels are set as NMOS transistors.
Referring to FIG. 1, a pixel 4 of the conventional organic light emitting display device includes an organic light emitting diode OLED and a pixel circuit 2 coupled to a data line Dm and a scan line Sn to control the organic light emitting diode OLED.
An anode electrode of the organic light emitting diode OLED is connected to an anode electrode, and a cathode electrode thereof is connected to a second power supply ELVSS. The organic light emitting diode OLED as above generates light having a predetermined brightness, corresponding to current supplied from the pixel circuit.
The pixel circuit 2 controls the amount of current supplied to the organic light emitting diode OLED by corresponding to a data signal supplied to the data line Dm when a scan signal is supplied to the scan line Sn. To this end, the pixel circuit 2 includes a second transistor M2 (that is, a driving transistor) connected between a first power supply ELVDD and the organic light emitting diode OLED, a first transistor M1 connected among the second transistor M2, the data line Dm and the scan line Sn, and a storage capacitor Cst connected between a gate electrode and a second electrode of the second transistor M2.
A gate electrode of the first transistor M1 is connected to the scan line Sn, and a first electrode thereof is connected to the data line Dm. A second electrode of the first transistor M1 is connected to one terminal of the storage capacitor Cst. Here, the first electrode is set as any one of a source electrode and a drain electrode, and the second electrode is set as the other electrode than the first electrode. For example, if the first electrode is set as a drain electrode, the second electrode is set as a source electrode. When the scan signal is supplied from the scan line Sn, the first transistor M1 connected to the scan line Sn and the data line Dm is turned on to supply the data signal supplied from the data line Dm to the storage capacitor Cst. At this time, the storage capacitor Cst is charged with voltage corresponding to the data signal.
A gate electrode of the second transistor M2 is connected to one terminal of the storage capacitor Cst and a first electrode thereof is connected to the first power supply ELVDD. A second electrode of the second transistor M2 is connected to the other terminal of the storage capacitor Cst and the anode electrode of the organic light emitting diode OLED. The second transistor M2 as above controls the amount of current flowing onto a second power supply ELVSS from the first power supply ELVDD via the organic light emitting diode OLED, corresponding to the voltage value stored in the storage capacitor Cst.
One terminal of the storage capacitor Cst is connected to the gate electrode of the second transistor M2, and the other terminal thereof is connected to the anode electrode of the organic light emitting diode OLED. The storage capacitor Cst as above is charged with voltage corresponding to the data signal.
The conventional pixel 4 as above supplies the current corresponding to the voltage charged in the storage capacitor Cst to the organic light emitting diode OLED, thereby displaying images having a predetermined brightness. However, the conventional organic light emitting display device as above has a problem that images having even brightness cannot be displayed due to deviation of threshold voltage.
When the threshold voltage of the second transistor M2 is actually set to be different for each pixel 4, each pixel 4 generates light having different brightness by corresponding to the same data signal so that images having even brightness cannot be displayed.